Aromatic polycarbonates belong to the group of engineering thermoplastics. They are distinguished by the combination of the properties transparency, heat resistance and toughness which are significant in engineering applications. High molecular weight linear polycarbonates are obtained by the phase boundary process by reacting the alkali metal salts of bisphenol A with phosgene in the two-phase mixture. Molecular weight may be controlled by the quantity of monophenols, such as for example phenol or tert.-butylphenol. These reactions virtually exclusively yield linear polymers. This may be demonstrated by end group analysis. Aromatic polycarbonates based on bisphenol A are in particular also used for the production of optical data storage media. They may, however, also absorb up to 0.34 wt. % of water, which has an unfavorable effect on the dimensional stability of data storage media. In other, especially external, applications, hydrolysis is a certain problem. The problem of hydrolysis resistance also arises in connection with use as steam-sterilizable medical articles.
On the basis of prior art, it has accordingly long been an object to provide a material which has the typical advantages of polycarbonate as an engineering thermoplastic, but which does not exhibit the above-stated disadvantages.
It has now surprisingly been found that certain polyformals and copolyformals are such materials.
Aromatic polyformals are also transparent thermoplastics which are synthesised from bisphenol structural units. In contrast with polycarbonates, however, the link between the bisphenol structural units does not consist of carbonate units, but instead of full acetal units. While in polycarbonate, phosgene serves as the carbonate source for the linkage, in polyformals, methylene chloride, for example, performs the function as the source of the full acetal linking unit during polycondensation. Polyformals may accordingly also be viewed as polyacetals.
In contrast with polycarbonates, aromatic polyformals may be produced in a homogeneous phase of bisphenol A and methylene chloride in the presence of alkali metal hydroxides.
In this polycondensation reaction, methylene chloride simultaneously acts as reactant and solvent. In the same manner as during polycondensation of polycarbonate, molecular weight may again be controlled by the quantities of monophenol used.
U.S. Pat. No. 4,374,974 has already described a process in which linear and cyclic oligo- and polyformals may be obtained starting from specific bisphenols after reaction with methylene chloride. One drawback of the materials obtainable in said process is their relatively high content of several percent of cyclic reaction products, which has a very disadvantageous impact on mechanical properties. Moreover, the described polyformals exhibit very unfavorable swelling characteristics in organic solvents, which make subsequent removal of the unwanted cyclic constituents virtually impossible.
DE A 27 38 962 and DE A 28 19 582 describe further and similar polyformals and the use thereof as coatings or films, with the above-stated disadvantages.
EP A 0277 627 describes polyformals based on specific bisphenols of the formula
and the possible use thereof as materials for optical instruments. In said application, substitution of the bisphenols on the aryl residues is described as mandatory in order to force the optical anisotropy of the polyformals into a range suitable for optical applications.
The polyformals previously described in the prior art and the properties thereof or their production processes are, however, unsatisfactory or have the disadvantage that there are limits to the achievable purities of linear polymer. The materials exhibit inadequate mechanical properties, which is, for example, manifested in elevated brittleness.
The prior art makes no disclosure as to the surprisingly and unusually low water absorption of polyformals, which makes polyformals of particular interest as engineering materials, in particular for optical data storage media.